The purpose of a communication system is to transmit information-bearing signals from a source (transmitter) to a destination (receiver) using a channel. The transmitter processes (modulates) the message signal into a form suitable for transmission over the channel. The receiver then demodulates the received signal to produce an estimate of the original message signal.
In any communication system, a key parameter which impacts system performance is the transmitter power. In a noise limited communication system, the transmitted power determines the allowable separation between the transmitter and receiver. The available transmitted power determines the signal-to-noise ratio at the receiver input which, for successful communication of information to occur, must exceed some prescribed threshold.
Another key performance criterion for certain communication systems relates to the number of simultaneous users that can be accommodated. An example of one well known system application is a cellular radio telephone system. Such systems are, typically, comprised of a number of base sites, each having a service coverage area, and a number of mobile or hand portable cellular telephones (hereinafter referred to as "subscribers"). The service coverage areas of base sites may be arranged to partially overlap in such a manner as to provide a substantially continuous coverage area in which a subscriber communication unit receiving service from one base site may be handed off to an adjacent remote site with no interruption in service. It is a key goal for a cellular communication system to effectively utilize the available spectrum so that as many users as possible can be accommodated.
Signal multiplexing permits the simultaneous radio transmission of signals from several message sources over a common spectral resource. Frequency division multiplex, time division multiplex, and mixtures thereof have traditionally been used for implementing cellular radio systems.
In a frequency division multiplex (FDM) system, the communication spectral resource is divided into several narrow frequency bands. For at least the time needed to communicate the desired traffic, one frequency division channel is occupied by the subscriber for communicating to the base site. Another frequency channel is used for traffic from the base site to the subscriber.
Time-division multiplex (TDM) systems are another type of multiple access communication system. In a TDMA system, the spectral resource is divided into repeating time frames each having a plurality of time slots or time division channels. Each time division channel is assigned to a different communication link. In this scheme, a portion of a subscriber's information occurs during an assigned slot of a frame. This is followed by one or more other time slots where information to or from other subscribers is accommodated. This process is repeated with received information being appropriately reconstructed at the receiver.
Both analog and digital transmission methods are used to transmit a message signal over a communication channel. Of recent, digital methods have become preferred due to several operational advantages over analog methods, including, inter alia: increased immunity to channel noise and interference; flexible operation of the system; common format for the transmission of different kinds of message signals; improved security of communications through the use of digital encryption; and increased capacity.
To transmit a message signal (either analog or digital) over a band-pass communication channel, the message signal must be manipulated into a form suitable for efficient transmission. Modification of the message signal is achieved by means of modulation and numerous suitable methods are well known in the art. Correspondingly, a receiver is required to recreate the original message.
Spread spectrum communication systems utilizing code division multiple access (CDMA) techniques can be used as multiple access systems like FDMA and TDMA systems. In a spread spectrum system a modulation technique is utilized in which the information is spread over a wide frequency band. The frequency band is much wider than the minimum bandwidth required to transmit the information being sent.
In a direct sequence CDMA system, communication between two communication units is accomplished by spreading each transmitted signal over a wide frequency band with a unique user spreading code. As a result, a multiplicity of transmitted signals share the same frequency. The ability of such a system to work is based on the fact that each signal is specially time and/or frequency coded to facilitate its separation and reconstruction at the receiver. Particular transmitted signals are retrieved from the communication channel by despreading a signal from the sum of signals in the communication channel with a known user spreading code related to the particular spreading accomplished at the transmitter.
In the digital direct sequence system, radio carrier modulation is performed after spreading the user's information with a digital code sequence whose bit rate is much higher than the information rate. A pseudo-random number (PN) is used as a code to "spread" the spectrum. The receiver, by utilizing the same known PN, can properly decode the received signal--even when corrupted with other user's spread signals--and reproduce the original information. The number of simultaneous users that can be accommodated in such a system is dependent on the amount of spectrum "spreading" that is implemented.
Another type of spread spectrum communication is "frequency hopping". In frequency hopping, the frequency of the carrier is shifted using a pattern dictated by a code sequence. The transmitter jumps from one frequency to another within some predetermined set. At the receiver, the hopping sequence for the desired user is known and allows tracking of the user's hopping transmissions. Periodically, more than one user's signal will fall on the same frequency thereby causing interference. Information coding techniques (error correction coding) are used to enable reconstruction of the original information even when a fraction of the transmitted bursts are lost. There are also time hopping and time-frequency hopping schemes whose times transmission are regulated by the code sequence.
Any of the multiple access systems can be utilized cellular radio communication systems. In cellular systems, several factors limit performance. Typically, in propagating through the channel, a transmitted signal is distorted because of nonlinearities and imperfections in the frequency response of the channel. Other sources of degradation are noise (thermal and man made) and adjacent and co-channel interference.
Besides the typical sources of degradation mentioned above, the majority of the noise associated with a received signal in a spread spectrum CDMA system comes from the other user's signals which are transmitted in the same frequency band but with unique user spreading codes. A noise power contribution to the desired despread signal exists for each of the other individual user's signals. The magnitude of the added noise is directly related to the received signal power of each of the undesired signals. An undesired received signal that comes in much stronger than the desired signal contributes excess noise. Therefore, it is desirable to dynamically adjust the power of all users in such a way that they are received with approximately the same power. In this manner, the number of users that can be simultaneously accommodated with the same spectrum resource is maximized.
In typical applications, to accomplish the needed power control, it would be necessary for the closest transmitters to reduce their power by as much as 80 dB when compared to the power of the furthest transmitters. This range in power control is extremely difficult to achieve and highly cost prohibitive.
Therefore, a means is needed by which communication using a spread spectrum format may be optimized without the requirement of an 80 dB power range.